1. Field
The invention relates to pickup tube scan correction devices, and particularly to a scan correction circuit and method for correcting the geometric, registration and shading errors generated in electron beam scanned and/or self-scanned image sensors.
2. Prior Art
In multiple tube video cameras, the images formed on the targets of the respective tubes must have correct spatial correspondence relative to each other to insure that the color component pictures subsequently displayed via a monitor, television receiver, etc., are in register, i.e., are spatially superimposed at all points. Consequently, the color analysis arrangements and the camera tubes must be very stable mechanically, and the scanning patterns traced on the tube targets must be as stable and identical as possible.
The characteristics of a camera tube that relate output signal and scene brightness, together with that of any gamma corrector used, must be such that over a large range of scene brightness an approximately linear relationship exists between a change of scene brightness and the corresponding change at the display. Therefore, in a camera employing several tubes and several gamma correctors, the combined characteristics of one tube and its associated corrector must be well matched to the others.
It follows that in prior art camera systems employing multiple pickup tubes the deflection yokes are first computer matched to provide yokes and tubes with similar characteristics, and thus scan raster geometries which are as matched as possible. In addition, one tube (for example, the green color channel tube) may be selected as a master tube/channel, and various analog waveform driving signals, specifically modified commensurate with the remaining geometric and registration scan errors of the (remaining) slave tubes/channels, are applied via suitable electronics, to thereby match the scan rasters of the slave tube, or tubes, with the master tube scan raster.
In such camera systems, it has been assumed that the original geometric and registration scan errors are corrected by using a sufficient number of waveshapes synchronous to the two scanning waveforms, which are then applied by adding them to, and/or modulating, the original scanning waveforms. However, all geometric and registration errors cannot be eliminated using the above techniques, since the scan errors only approximate the two orders of sawtooth and parabola waveforms, which are commonly used for scan correction.
In addition, camera systems employing a manual set-up use potentiometers located on a control panel which is integral with the central control unit (CCU). The CCU is in turn remote from the camera head unit and is coupled thereto via multiplexed parallel conductors in a cable, or the like. The analog error correction signals are encoded, multiplexed and decoded, thereby requiring a considerable amount of analog circuitry, which generates considerable drift and thus stability problems. The potentiometers must be continually read and readjusted, whereby the camera head system is continually dependent upon the signals from the CCU for proper operation.
By way of example only, typical of camera systems which employ the above correction techniques to provide scan and shading error corrections are the broadcast color cameras, Models BCC-1 and BCC-10, shown in the Ampex "Service Data Package" Manual No. 1809326-01, Ampex Corporation, Redwood City, California.